Tissue engineering approaches for driving stem cells toward spatially-organized multi-tissue units of the musculoskeletal system, such as muscle-tendon-bone (MTB), will require spatial control of differentiative cues provided by various components of tissue-engineered constructs, including their: biochemical elements;scaffold material composition and structure;and, biomechanical interactions. Current toolsets to aid in the early stages of discovery, design, and implementation of such complex, multi-variable constructs are either non- existent or severely limited in their capabilities to incorporate spatial control of those biochemical elements provided by exogenous paracrine signaling factors (PSFs). To address this need, we propose a novel PSF biopatterning technology that will enable the creation of persistent, spatially-defined patterns of PSFs organized in multiple neighboring regions of a scaffold, where each region targets a different phenotype to be induced. This capability will be unique because it will enable an exogenous or endogenous stem cell population exposed to a PSF-patterned construct to be driven toward multiple differentiative fates simultaneously in register to these patterns, at sub-millimeter resolution, to form neighboring multi-phenotype groupings within the same construct, both in vitro and in vivo. Pattern designs for an MTB will first be determined with the aid of a systematic design methodology applied to in vitro studies to identify a minimum set of spatially-patterned PSF cues out of a very large number of design possibilities, and then the resulting highest ranking designs will be validated in vivo for driving tissue phenotype formation in an ectopic subcutaneous mouse model. As an additional in vivo validation PSF patterned constructs will be implanted into a mouse Achilles tendon wound model to initiate site-specific host response, and histologically assessed for tissue phenotype expression in register to patterns applied. PUBLIC HEALTH RELEVANCE: New tissue engineering therapies are needed to address the growing demand to repair multi-tissue structures of the musculoskeletal system, such as interconnected bone-tendon-muscle units that are diseased or injured. This becomes an even greater challenge because of the need to spatially control multiple differentiation fates simultaneously, including multi-unit tissue interfaces, within the same intercommunicating pericellular environment. There is an unmet need for new tissue-engineered construct technologies and design methodologies that will enable a stem cell population to be driven toward neighboring regions of different differentiation fates in each region, in vitro and in vivo. We propose to develop and demonstrate a spatial patterning methodology that uses a limited number of exogenous signaling molecules, patterned in scaffolds, to direct stem cells in the musculoskeletal system down multiple neighboring and intercommunicating differentiation fates as a first order model of multi-tissue formation and interaction. Engineered spatial patterning will provide new insights about multi-tissue formation, with the long-term goal to use patterned constructs to improve clinical outcomes of musculoskeletal-related treatments, which represents an estimated annual direct and indirect cost of $510 billion, in terms of 2004 dollars, or 3.1 % of the GDP in the US alone.